1. Field of the Invention
The present invention relates to a fiber amplifier for amplifying the magnitude of an input optical signal in an optical communication system, and more particularly, to a gain-clamped optical amplifier having uniform gain characteristics in spite of change in the magnitude of the input optical signal.
2. Discussion of Related Art
In general, an optical amplifier such as a semiconductor optical amplifier and a fiber amplifier is an optical device for amplifying the magnitude of an optical signal which must be used in order to compensate for optical loss generated in optical fiber for transmission or various optical devices in the fields of optical transmission and optical network.
However, according to the conventional optical amplifier, the degree of amplification varies with the magnitude of an input optical signal, such that communication quality deteriorates in the optical network. Therefore, in order to solve the problem, a gain-clamped fiber amplifier has been developed.
An all-optical gain-clamped fiber amplifier of optically clamping gain using laser resonance does not need a complicated signal processing process for clamping gain. When loss and gain generated by a cavity are the same, laser oscillation occurs. When the laser oscillation occurs, the magnitude of the population inversion of a gain medium is fixed.
Since the gain of the fiber amplifier is proportional to the magnitude of the population inversion and the length of the gain medium, when the laser oscillation occurs, the gain of the amplifier is clamped. When an optical signal is input to the fiber amplifier whose gain is clamped due to the laser oscillation, gain is maintained uniform regardless of the magnitude of the optical signal when the input optical signal is weak, and the laser oscillation stops and the gain-clamped characteristic of the fiber amplifier disappears when the magnitude of the input optical signal increases.
FIG. 1 illustrates an example of the conventional gain-clamped fiber amplifier that uses the laser oscillation.
Referring to FIG. 1, when optical fiber is used as a gain medium 1, pumping light is supplied from a laser diode (LD) 3 through a wavelength division multiplexed (WDM) optical coupler 2. An optical attenuator (ATT) 5, optical isolators (ISO) 6, and a backlit optical filter (BPF) 7 form a ring cavity for the laser oscillation by optical couplers 4 of input and output stages.
The backlit optical filter 7 controls the wavelength at which the laser oscillation occurs. The optical isolators 6 let oscillation performed in one direction in the ring cavity. The optical attenuator 5 controls the optical loss of the cavity to control the gain of the amplifier.
In the fiber amplifier structured as illustrated in FIG. 1, the magnitude of an input optical signal and the magnitude of a laser optical signal that oscillates inside are complementary to each other. That is, the magnitude of the laser oscillation light increases when the magnitude of the input optical signal is weak and the magnitude of the laser oscillation light is reduced when the magnitude of the input optical signal increases such that amplification ratio is maintained uniform even if the magnitude of the input optical signal changes to some extent. At this time, when the magnitude of the input optical signal increases to no less than a uniform level, the laser oscillation stops and the gain of the fiber amplifier is reduced like the gain of a common fiber amplifier.
However, according to a gain-clamped fiber amplifier that uses the laser oscillation, when the magnitude of the input optical signal changes, the magnitude of the amplified optical signal temporarily fluctuates due to relaxation oscillation due to the laser cavity. Temporary change in the magnitude of the optical signal affects the bit error rate of transmitted data. Also, since a relaxation oscillation frequency is determined in accordance with the characteristic of the gain medium and the length of the cavity, it is difficult to manufacture the fiber amplifier and light transmission speed and optical signal processing speed are restricted.